A fine metal, for example, a fine metal having a surface structure on a nanometer scale and a metallic fine particle of a nanometer size shows a characteristic optical response (optical absorption) called “localized (surface) plasmon resonance absorption” in a specific wavelength range according to its shape and size. Examples of metal showing the localized plasmon resonance absorption include noble metals such as gold, silver and platinum, but it is important that, even if the same kind of metal is used, plasmon resonance absorption wavelengths are different depending on the size and shape. Making full use of such properties, the application to various kinds of optical devices (for example, optical filters) of fine metal or metallic fine particles described above is expected.
The localized plasmon resonance absorption also includes an important application. Intensity of the optical response (light emission or Raman scattering) of molecules adsorbed onto a metal showing plasmon resonance is significantly increased (104 times or more) by an interaction between the molecules and surface plasmon. That is, a metal structure having a fine metal showing plasmon resonance prepared on a substrate will function as a high sensitivity sensor device for a molecular system, and research and development of application to this field is actively conducted.
When a metal structure having a substrate on which a plurality of metallic fine particles having plasmon resonance absorption are arranged is applied to various kinds of optical devices or high sensitivity sensors, first, it is important to control a position of plasmon resonance absorption wavelengths and polarization selectivity of incident light to be absorbed. Characteristics of a plasmon resonance absorption band (such as resonance absorption wavelengths, spectral line shapes and polarization selectivity) are sensitive to orderliness of the size and shape of each metallic fine particle, and also sensitive to orderliness of the distance between metallic fine particles arranged on the substrate, distribution (variability) of the distance and directional properties of arrangement. Therefore, a nanostructure of metallic fine particles and arrangement of a plurality of metallic fine particles on the substrate must be precisely controlled in order to control optical response characteristics of the surface plasmon. If the metallic fine particles are rod-shaped metallic fine particles (metallic nano-rods), the shape thereof can be specified by, for example, the aspect ratio (ratio of the major axis length and minor axis length).
For example, Patent Document 1 discloses an optical filter consisting of an acrylic resin film with thickness of several μm into which gold nano-rods is distributed, the rods having an anisotropic shape whose average length of minor axis is 10 nm and that of major axis is 100 nm or less. In this case, the aspect ratio of the gold nano-rod is 10 or less. It has been confirmed that an optical filter prepared in this way has light transmittance of 15 percent or less in all wavelength ranges of 800 nm to 1000 nm and 1200 nm to 1600 nm due to plasmon resonance of gold nano-rods and thus operates as an excellent optical filter in the near-infrared region.
Furthermore, according to Patent Document 2, resin material containing gold nano-rods (composition containing gold nano-rods) having a small variation coefficient (20 percent or less) of major axis and minor axis lengths (that is, sizes of the gold nano-rods are roughly the same), prepared through a reduction of metallic ion shows relatively sharp wavelength characteristics of plasmon absorption (that is, wavelength selectivity is high).
Patent Document 1: Japanese Patent Application Laid-Open No. 2003-315531
Patent Document 2: Japanese Patent Application Laid-Open No. 2004-292627